The invention relates to x-ray diagnostic apparatus utilizing a video camera; at least one image memory; a subtraction stage for producing a difference signal representing the difference between a stored video signal and a video signal which chronologically follows the stored video signal; a circuit for evaluating the time variation in the difference signal and a video monitor for displaying the processed x-ray images. The evaluation circuit in this diagnostic apparatus comprises a time memory that can be used to store information about the chronological progression of the contrast medium. Subtraction images are used for x-rays, especially for selective angiography, to make clearly visible blood vessels that are difficult to recognize in a normal x-ray picture or are located behind bone structures.
The U.S. Pat. No. 4,398,213 discloses x-ray diagnostic apparatus in which an evaluation circuit recognizes the chronological occurrence of changes in the differential image and reads the respective pieces of time information into a time memory. These pieces of time information consist of numerical values, and the magnitude of the stored numerical values characterizes the chronological occurrence of the maximum of the contrast medium flow. A color converter coupled to the memory converts these numerical values into video signals of different colors, which are then reproduced on a color video monitor. The progression of the contrast medium with respect to its position and the chronological occurrence of its maximum can be observed on the monitor, with the aid of this color image. For example, a blood vessel through which the contrast medium is flowing is shown in color and the red image dots may signal an early point in time, while the blue image dots represent the end of the contrast medium flow. Thus only the total, colored image can be seen upon the completion of the x-ray photography. A repeated representation of the flow of the contrast medium is impossible. Furthermore, a color monitor must be available in each case for the correct evaluation of such a subtraction image.